THE NATURE OF THE CONDUCTANCE INCREASE INDUCED BY FILIPIN IN CHOLESTEROL-CONTAINING PLANAR LIPID BILAYERS

The effects of the polyene antibiotic filipin on the conductance and permeability of planar lipid bilayers were investigated under voltage-clamp conditions. The membrane conductance of lipid bilayers containing no cholesterol was not affected by filipin. In the presence of cholesterol containing lipid bilayers, filipin induced a 10(4)-10(5)-fold increase in transmembrane conductance. This conductance increase was dependent on the ionic species present in solution, decreasing in the following order: G(CsCl) > G(NaAc) > G(KCl) > G(CaCl2) > G(Na2SO4) > G(BaCl2) > G(MgCl2. Reversal potential measurements in simple biionic conditions revealed the following relative permeability sequence: P(K) > P(Cl) > P(Na) almost-equal-to P(ac) almost-equal-to P(Ba) > P(Cs) > P(Mg) almost-equal-to P(Ca) > P(sulphate). The filipin-sterol mediated increase in membrane conductance was independent of the membrane potential. The increase in membrane current following a step alteration in membrane potential occurred instantaneously and had no dependence on the previous value of the holding membrane potential. We propose that the filipin-sterol complex forms ion channels in lipid membranes. These channels are found in a single configuration (open state) and select preferentially monovalent cations or anions over divalent ions. Our experimental results are discussed in relation to the effects of other polyene antibiotics on the membrane permeability, and also in relation to experimental problems previously reported with the use of filipin in planar lipid bilayers.